1. Field of the Invention
The present invention relates generally to antennas. More specifically, the present invention relates to a selectively coupled two-piece antenna for mobile phones.
2. Description of the Related Art
Personal communications devices such as mobile phones have become increasingly common in the past few years. Whip antennas are commonly used in mobile telephones. A shortcoming of whip antennas is that they often catch on things and become damaged. In order to prevent such damage, many whip antennas are designed to be retractable into the mobile telephone casing. Thus, the typical mobile phone, whether it be for use in a cellular system or a satellite telephone system, has a whip antenna that is retractable into the casing when not in use. A user desiring to send or receive a call will extend the antenna from the casing. Similarly, when a user is not engaged in a call, the antenna can be retracted into the casing.
For many mobile phones, the center of its antenna is aligned with a user""s head and/or hands during operation. Due to the standing wave patterns in a typical whip antenna, the user""s head and/or hands tends to obstruct signals that are transmitted and received through the whip antenna. This obstruction is also known as shadowing and tends to degrade mobile phone performance.
As technology advances, the size of mobile phones is continually reduced. As a consequence of this reduction in size, small sized mobile phones contain less space to accommodate whip antennas. Thus, retractable whip antennas that are used with such small sized mobile phones have also by necessity become shorter. Unfortunately, shorter whip antennas are less able to avoid the signal shadowing effects described above.
Some mobile phones employ a helical antenna instead of a whip. For these antennas, a helix protrudes slightly from the phone casing and is usually fixed. Therefore, it is neither retractable nor extendable. User convenience is a motivation behind the use of fixed helical antennas. If a user does not have to extend and retract the antenna, operation becomes simpler from the user""s perspective. Also, a phone employing a fixed helical antenna can be made somewhat more compact since the phone""s casing does not have to accommodate the length of a retracted whip. However, the shadowing problem describe above is often exacerbated with a helix.
Many phones today use a combination of a helical antenna and a whip antenna. One such approach involves a configuration where a helix is disposed on the exterior of the casing and an extendable whip passes through the center axis of the helix.
Another approach involves placing a helix on the distal end of the whip. When the whip is retracted, only the helix protrudes from the casing. In a first variation of this approach, the whip and helix are electrically disconnected in both the extended and retracted positions. In a second variation of this approach, the whip and helix are electrically connected in the extended position, but electrically disconnected in the retracted position.
Examples of such known devices are described in the following U.S. patents:
U.S. Pat. No. 5,426,440 to Shimada et al.,
U.S. Pat. No. 5,594,457 to Wingo,
U.S. Pat. No. 5,650,789 to Elliot et al., and
U.S. Pat. No. 5,717,408 to Sullivan et al.
Many mobile phones employ digital circuitry that generates signals having high frequency harmonics. In certain cases, these harmonics can fall within a mobile phone""s receive band. When an antenna is retracted, it is often in close proximity to such digital circuitry. As a result of this proximity, the portion of the antenna that is in the mobile phone""s casing can receive these signals and send them to components within the mobile phone designated for the reception of communications signals. This phenomena is known as self-jamming, and it intensifies as mobile phones become smaller in size. Self-jamming causes interference with radio frequency (RF) communications and degrades mobile phone performance.
Self-jamming can be mitigated by shielding the electronic components that generate high frequency harmonics in a grounded conductive can. Alternatively, self-jamming can be mitigated by shielding the retracted portion of the antenna with a conductive tube that is grounded. However, these solutions are costly and involve several mechanical and spatial constraints. Another approach involves grounding the antenna when it is in its retracted position. This grounding creates a high input impedance for the antenna and requires the implementation of matching circuitry to match the antenna impedance to the impedance of other RF components. This matching circuitry consumes space in the mobile phone and increases the phone""s cost.
As a result, it has been recognized that there is a need for a mobile phone antenna that reduces shadowing caused by users when extended and provides a compact, cost effective approach to the mitigation of self-jamming when retracted.
The present invention is directed to a selectively coupled two-piece antenna for use in a mobile phone that has a casing and RF communications circuitry. The selectively coupled two-piece antenna comprises a composite radiator that is selectively extendable from and retractable into the casing and a communications interface that is connected to the RF communications circuitry. The composite radiator has first and second radiating elements, and a connecting element.
When the composite radiator is extended, the connecting element connects the first and second radiating elements. In this position, the communications interface connects the RF communications circuitry to the first and second radiating elements. Thus, the RF communications circuitry transmits and/or receives RF signals through both the first and second radiating elements as a top loaded antenna.
However, when the composite radiator is retracted, the connecting element electrically isolates the first and second radiating elements. In this position, the composite radiator contacts the communications interface so that the first radiating element is electrically connected to the RF communications circuitry. Thus, in this position, the second radiating element is electrically disconnected from the RF communications circuitry. Therefore, the RF communications circuitry exchanges signals with only the first radiating element when the composite radiator is retracted.
Another advantage of the present invention is the elimination of self-jamming interference when the composite radiator is retracted.
Further features and advantages of the invention, as well as the structure and operation of various embodiments of the invention, are described in detail below with reference to the accompanying drawings.